Coalfield

The South Wales Coalfield lies beneath a plateau supported by layers of hard sandstone in the upper Coal Measures, the Pennant Sandstone. The quarry (317 189) on the skyline above Caerphilly is in Pennant Sandstone that also makes most of the scarp faces that loom above the valleys of the Coalfield. The valleys cut down along north-northwest trending faults through the Pennant Sandstone to the Coal Measures, so mines and their towns are in the valley floors.

Fig. 6. Geologic map of the Valleys showing the eastern part of the Coalfield syncline.

The A40 crosses the northern rim of the mountains between Abergavenny and Llandeilo. The M4 follows the southern boundary. The A465, the Head of the Valleys road, runs along the northern ends of the deep valleys cut into the plateau, and the A470 crosses the Coalfield between Cardiff and Brecon.

The sedimentary rocks of the Coalfield are deltaic sandstones with coal formed from the forests and swamps that populated the deltas. Alternations of coal beds with sand and mud record the shifting floodways. Then the Pennant sands covered it all. The upper part of the Pennant Sandstone contains minor coal beds that are still mined in the hills above Ystrad Mynach (314 194).

The mines are nearly all closed except the large open-cast mines near Hirwaun (295 205) and at Seven Sisters (282 208).

The mines, when working, had their pitheads in the valleys and the towns clustered around them.

The A470

The A470 is the main road across the Coalfield. It leaves the M4 at Exit 32, passes through Merthyr Tydfil and the Brecon Beacons National Park, and thence to Brecon. The outcrops on the valley sides expose the Pennant Sandstone. The rocks of the Coalfield form a broad syncline (Fig. 8), with the youngest rocks in the centre.

You can see more rocks if you leave the A470 at Abercynon (308 196) and follow the A4059 to Aberdare (300 202). Then take the A4233 to Rhondda (297 196). The road climbs steeply through the Pennant Sandstone, passing many outcrops as it climbs over the divide between the valleys. The shortcut between Tylorstown (301 195) and Rhondda provides a fine view of the Rhondda valley, one of the famous sites in the Coalfield. It inspired Richard Llewellyn's book How Green Was My Valley.

From Treherbert (294 198), at the head of Rhondda Fawr, the road climbs rapidly up through the non-productive part of the Coal Measures, and into the Pennant Sandstone (Fig. 9). High on the west side of the valley above Treherbert is a small glacial bowl, or cirque, with irregular piles of glacial till below it. These features are the result of a small glacier that filled the cirque during the last stage of the ice age.

The A4061 climbs onto the moor and provides fine views of the Rhondda and the Taff Vale. Many road cuttings show excellent exposures of the Pennant Sandstone.

The north-facing cliff at the pass into the Aberdare valley is the headwall of a fine glacial cirque. The melting glacier left a pile of till at the bottom of the cirque that now impounds a lake, Craig y Llyn. The small glaciers that eroded this cirque and the one at Treherbert are the same age as the youngest glaciers in Snowdonia, about 10,000 years ago. The road between Craig y Llyn (292 203) and Hirwaun (295 205) passes a working open-cast coal mine. Return along the A465 to Merthyr Tydfil to rejoin the A470 northward to Brecon.

Merthyr Tydfil (305 207) lies on the Lower Coal Measures, and also had important iron mines. This was once the steel capital of Wales. Prominent road cuttings north of Merthyr Tydfil are in sandstone of the Millstone Grit beneath the Coal Measures. The beds dip gently to the south so when you travel northwards the road cuts down into the Carboniferous Limestone and then into the Old Red Sandstone in the Vale of Neath.

The A470 crosses a pass into the Usk valley over one of the hard layers of the Upper Old Red Sandstone. Then it drops through thick sandstones into the softer mudstones of the lower part of the Old Red Sandstone.

The rounded moors of the Brecon Beacons (Fig. 10) near the pass are in the resistant sandstones of the upper part of the Old Red Sandstone. Many small quarries and road cuttings expose the red sandstone along the road north of the pass (Fig. 11).

Geophysicists removed cores from some of these quarries to measure the direction of the magnetic field in the cores. The inclination of the magnetic field in the rocks depends on the latitude at the time of deposition. It seems that Wales was twenty degrees south of the equator when flash floods laid down these red sandstones.

Fig. 11. Quarry on the A40 north of the Brecon pass cut into thick beds of Devonian red sandstone. Geophysicists drilled the holes in the rock to measure the magnetic field direction.

Brecon is in the Usk Valley, where the river eroded into the softer sandstone and mudstone in the lower part of the Old Red Sandstone. The A470 continues north to Conwy on the north coast.

Hirwaun, on the A465, is in a wide valley carved into the soft Coal Measures that come to the surface on the northern limb of the broad Coalfield syncline. The rocks dip gently down to the south, so you pass into older rocks as you drive north on the A4059, through the Coal Measures down into the sandstone of the Millstone Grit. None of these rocks are well exposed around Hirwaun. The first rocks you see along the A4059 north of Hirwaun is Carboniferous Limestone that lies beneath the Millstone Grit. It first appears as limestone blocks in fields south of Penderyn (295 209), and then in road cuttings near a quarry north of the village.

North of Penderyn, near the turnoff to Ystradfellte, a prominent low scarp sheds large white blocks onto the fields below, giving them a decidedly untidy look (Fig. 12). The rock in the scarp is the lowest bed of the Millstone Grit, pure sandstone crowded with pebbles of white quartz. Several beds of Carboniferous Limestone beneath the white sandstone give the landscape a ribbed appearance.

Fig. 12. A ridge of Millstone Grit, here a white quartzite, lying above ribs of Carboniferous Limestone. Near Ystradfellte, north of Penderyn on the A4059.

The Ystradfellte caves (293 213) are a popular attraction. Slightly acidic rain water trickling through the limestone dissolved the rock to create the caves. Like all limestone caverns, they follow fractures and a less resistant layer of limestone.

North of the turnoff to the caves, the A4059 runs northeast beneath the scarp of the Millstone Grit with the ribs of limestone clearly visible beneath the white sandstone. Then the road turns north across the moors on the Old Red Sandstone, where it passes very few bedrock outcrops. A small quarry beside the road (299 214) exposes thin layers of siltstone that show some cross bedding. The Brecon Beacons lie to the north, capped by more massive sandstone.

The Heads of the Valleys road, the A465, runs between Abergavenny in the east and Swansea in the west. The road climbs steeply out of Abergavenny through the Clydach Gorge, first through Carboniferous Limestone, then higher through the Millstone Grit and into the Coal Measures (Fig. 14).

Fig. 14. Left: Clydach Gorge, the A470 east of Brynmawr. Shale and sandstone of the Coal Measures.

The most obvious outcrops are of thick sandstone spectacularly exposed in road cuttings, on the valley walls, and in a quarry (Fig. 15) just east of Brynmawr (310 212). They are typical deltaic sands that are interbedded with marsh laid shale deposits exposed in the road cuttings on the A465. The Coal Measures lie above these thick sandstones.

Fig. 15. Right: Black Rock Quarry. East of Brynmawr. Thick sandstone in the Coal Measures. The view is about 20 metres wide.

The largest expanse of the Lower and Middle Coal Measures is along the broad valley at the Head of the Valleys. Brynmawr (316 212) and Merthyr Tydfil (305 206) stand on the most productive layers of coal. Merthyr Tydfil, once the centre of the Welsh steel industry, produced both coal and steel. The iron ore came from clay ironstone beds in the Coal Measures. Those mines are all closed.

West of Merthyr Tydfil are a few road cuttings in thin-bedded sandstone, some with very thin coal beds. South of the road are the scarps of the Pennant Sandstone that lie above the productive Coal Measures. A large open cast coal mine still operates at Hirwaun (296 205).

West of Hirwaun, the road turns southwest along the Vale of Neath. The valley follows a belt of several ancient faults that trend northeast. The Hercynian collision jostled these faults and produced tight folds in the rocks above. The complex of faults and local folds are locally known as disturbances because they disturb the otherwise gently dipping beds (Fig. 17).

The best place to see the Neath disturbance is northeast of Glyn Neath at the village of Pontneddfechan (290 207). The layers of sandstone in the hills to the south lie flat but rumple into a tight fold at the disturbance. At Craig-y-Dinas, a nature reserve east of the village, the limestone dips steeply to the north. It is well exposed in a quarry of unusual black limestone, which provided polished marble for many decorative fireplaces. A large fold appears along a nature trail, about a half kilometre northeast.

Fig. 16. Dinas Rock, Craig-y-Dinas, Pontneddfechan. The Neath disturbance tilted the Carboniferous Limestone steeply down to the north.

Fig. 17. Section across Dinas Rock (Fig. 16) showing the narrow band of rock affected by the Neath disturbance.

The A4067 follows the broad Swansea Valley through the Pennant Sandstone. The valley contains some coal seams in its upper part. Towns such as Ystalyfera (276 208) cluster around the old mines.

An alternative route follows the Vale of Neath on the A465, then along the A4109 past the open-cast mine at Seven Sisters. The Cefn Coed (278 203) coal museum about five kilometres north of the intersection with the A465 contains exhibits illustrating mining methods and the history of mining.

North of Ystalyfera on the A4067, the beds dip gently down to the south, so the road passes northward into older rocks. At Abercaf (282 212), in the Tawe Valley, the white Millstone Grit caps the mountain to the north; Carboniferous Limestone lies beneath it in the lower slopes.

The hills on either side of the road north of Abercaf show prominent ribs of Carboniferous Limestone, capped by the white Millstone Grit. The best place to see these rocks is along the road to the

Fig. 19. Pennwyllt quarries, near Abercaf. White quartzite of Millstone Grit forms a prominent scarp. The darker grey cliffs are the underlying Carboniferous Limestone.

Pennwyllt quarries (Fig. 19.), east of Craig-y-Nos (284 215). Abandoned quarries in Carboniferous Limestone dot the hills; some have white quartzite and conglomerate of the Millstone Grit above the limestone. It is interesting to speculate what change caused rivers to deposit pure quartz sand and pebbles of vein quartz into the clear and shallow seas where the limestone was accumulating.

Fig. 20. Left: Carboniferous Limestone with sea lily fossils, crinoids. The black layer is chert. The view is 50 centimetres wide. Pennwyllt quarries.

A minor road between Pen-y-Cae (284 213) and Coelbren (286 212) passes the Henryd waterfall (2853 2119) (Fig. 22), a ward of the National Trust. The waterfall is about 30 metres high, the highest in South Wales. The water drops over Farewell Rock, which is the miners' term for the sandstone below the lowest coal seam.

North of the commercial Carboniferous Limestone caverns at Dan-yr-Ogof (284 216) the A4067 passes into the Old Red Sandstone below the limestone. The limestone outcrops make distinct ribs above the road.

A road cutting about two kilometres north of the caves at Dan-yr-Ogof shows tilted layers of sandstone of the Devonian Old Red Sandstone. The beds have ripple marks on their upper surfaces, sculpted by water flowing over the loose sand on the alluvial plain. The nearby hills have the reddish tinge typical of soils developed on the Old Red Sandstone. The subdued Old Red Sandstone landscape continues north to the intersection with the A40 and A470. Ribs on the hills show the position of the thicker layers of sandstone, but there are few outcrops.

At Llangadog (271 228), the A4069 cuts south from the A40 between Llandovery and Llandeilo, and crosses Black Mountain to Brynamman. This road passes more bedrock outcrops and has better views than the main road, the A4067, between Swansea and Brecon.

A road cutting opposite the chapel in Llangadog exposes the oldest rocks in the region, the Ordovician slate of the Welsh Basin. About five kilometres south of the A40, the road passes through a narrow valley where the river has incised a small gorge beneath the level of the road. You can see the gorge at the bridge by the Three Horseshoes pub. The rocks are siltstone with a poor slaty cleavage -- the typical Silurian rock of central Wales.

Fig. 23. A4069 south of Llangadog, just south of the first hairpin bend. Road cutting in sandstone of the Devonian Old Red Sandstone.

Farther south, the road passes into younger Devonian rocks, which look very like the Silurian siltstone beneath. As the road climbs Black Mountain, you see a marked change in the rocks and in the landscape. Large boulders litter the moors, and a few road cuttings show glacial till plastered onto the mountain as the ice melted. Gullies eroded through the till reveal that it lies on typical red sandstone of the Old Red Sandstone.

The top of the Old Red Sandstone is well exposed at a car park (273 219) (Fig. 24) about half way up the mountain. Fine views of the Towy Valley stretch to the north. Rocks around the viewpoint are coarse red sandstone that contains many pebbles of white vein quartz. This bed is the same quartz conglomerate that caps the plateau of the Forest of Dean to the east.

Fig. 24. The topmost layer of the Devonian Old Red Sandstone exposed at the viewpoint on Black Mountain.

Higher up the mountain, the road climbs into the much-quarried Carboniferous Limestone. Where the road crosses the divide into the Loughor valley, it passes into the sandstone of the Millstone Grit that lies above the limestone. These sandstones are not as white as the beds around the Pennwyllt quarries to the east. The road then descends to Brynamman (272 214), and passes into younger rocks, the Coal Measures. Coal was once mined at Brynamman, but all that now remains is the spoil tip. The colliery has gone.

The A483 leaves the M4 at Exit 49, and follows the River Loughor to Ammanford (263 212). North of Ammanford, the A483 crosses a subdued and pastoral landscape lying upon unproductive Coal Measures. At Pentre-Gwenlais (262 216) the road cuts through a ridge of Carboniferous Limestone that dips to the south because it is on the north side of the Coalfield synclinal trough. Quarries in the limestone lie west of the village. The grazing land Between Pentre-Gwenlais and Llandeilo hides a thick section of Devonian Old Red Sandstone and Silurian siltstone.

ROADSIDE GEOLOGY OF WALES

THE SOUTH WALES COALFIELD